1. Field of the Invention
The present invention relates generally to a method for eliminating internal reflection of range finding systems, and more particularly to a method for eliminating internal reflection occurring in a laser sight.
2. Description of the Prior Art
A conventional laser sight generally comprises a range finding system comprising an object lens, an eyepiece, an erecting system, a laser transmitter, a transmitting prism, a laser receiver, and a receiving prism. An example of the conventional laser sight is disclosed in U.S. Pat. No. 5,771,623. In operation, the laser transmitter transmits, at a first time point, a transmission light beam to an object, and then the laser receiver receives a reflection light beam that is reflected by the object at a second time point. The distance between the laser sight and the object can be obtained by multiplying the light speed with the time difference between the second and first time points.
FIG. 1 of the attached drawings shows a range finding system 90 of the conventional laser sight. The laser range-finding system 90 comprises a laser transmitter 93, a transmitting prism 94, an object lens 95, a receiving prism 96, and a laser receiver 97. The laser transmitter 93, the transmitting prism 94, and the object lens 95 are arranged on a transmitting beam path 91. The object lens 95, the receiving prism 96, and the laser receiver 97 are arranged on a receiving beam path 92. The laser transmitter 93 transmits a light beam to the transmitting prism 94. The prism 94 redirects the light beam toward an object (not shown) through the object lens 95. The light beam is then reflected by the object back to the receiving prism 96 through the object lens 95, and is redirected to the laser receiver 97 by the receiving prism 96. However, since the transmitting beam path 91 and the receiving beam path 92 are coincident with each other (or in other words, they are coaxial), internal reflection of the transmitting light occurring along the transmitting beam path 91 is also received by the receiver 97. For example, when the light beam generated by the laser transmitter 93 is shot at the object lens 95, an unwanted internal reflection signal will be generated, which, by traveling along the receiving beam path 92, is received by the laser receiver 97. Since the receiver 97 detects both the internal reflection and the reflected beam from the object to be measured, interference of the reflected lights occurs and the laser receiver 97 works improperly. This is particular true when the laser sight is used for close-quarters range finding, because the signal reflected by the object and the internal reflection signal are both very strong, causing a large electrical current in a light sensitive element (not show) of the laser receiver 97. The large current may easily result in a circuit saturation, which prevents the laser sight from working properly, or even damages the laser sight.
Hence, a method for eliminating internal reflection in a range finding system and a related signal processing method, to protect the laser sight, is required to overcome the above-mentioned disadvantages of the prior art.